Liquid chromatography and mass spectrometry have proven powerful analytical tools in identifying molecular components of our world. Liquid chromatography is a fundamental separation technique. Mass spectrometry is a means of identifying "separated" components according to their characteristic "weight" or mass-to-change ratio. The liquid effluent from LC is prepared for ionization and analysis using any of a number of techniques. A common technique, electrospray, involves spraying the sample into fine droplets.
Early systems for electrospray LC/MS utilized flow splitters that divided the HPLC (high performance liquid chromatography) column effluent. As a result of the effluent splitting, only a small portion, typically 5-50 micro liters per minute, was introduced into the "spray chamber". The bulk of the column effluent did not enter the spray chamber but went directly to a waste or fraction collector. Because electrospray/mass spectrometry (ES/MS) generally provides a concentration sensitive detector, it was not necessary to analyze the entire column effluent flow to obtain sensitive results. Results obtained by splitting are comparable in sensitivity to those obtained by introduction of the entire column effluent flow into the spray chamber (assuming equal charging and sampling efficiencies).
Such low flow rates enabled generation of an electrosprayed aerosol solely through the manipulation of electrostatic forces. However, the use of flow splitters gained a bad reputation due to potential plugging problems and poor reproducibility.
Newer electrospray systems generate a charged or ionized aerosol through the combination of electrostatic forces and some form of an assisted nebulization. Nebulization is the process of breaking a stream of liquid into fine droplets. Nebulization may be "assisted" by a number of means, including but not limited to pneumatic, ultrasonic or thermal assists. The assisted nebulization generates an aerosol from the HPLC column effluent, while electric fields induce a charge on the aerosol droplets. The charged aerosol undergoes an ion evaporation process whereby desolvated analyte ions are produced. Ideally, only the desolvated ions enter the mass spectrometer for analysis.
A challenge in any assisted nebulizer system is designing the vacuum system leading to the mass spectrometer such that desolvated ions enter, but relatively large solvated droplets present in the electrosprayed aerosol are prevented from entering. Several design approaches are currently in use, but none has solved all the challenges. None of the assisted nebulization methods currently practiced provide reliable sensitivity along with robust instrumentation.
In conventional electrospray/nebulization mass spectrometry systems, the electrosprayed aerosol exiting from the nebulizer is sprayed directly towards the sampling orifice or other entry into the vacuum system. That is, the electrosprayed aerosol exiting from the nebulizer and entry into the vacuum system are located along a common central axis, with the nebulizer effluent pointing directly at the entry into the vacuum system and with the nebulizer being considered to be located at an angle of zero (0) degrees relative to the common central axis.
One previous approach directed at improving performance adjusts the aerosol to spray "off-axis". That is, the aerosol is sprayed "off-axis" at an angle of as much as 45 degrees with respect to the central axis of the sampling orifice. In addition, a counter current gas is passed around the sampling orifice to blow the solvated droplets away from the orifice. The gas velocities typically used generate a plume of small droplets. Optimal performance appears to be limited to a flow rate of 200 microliters per minute or lower.
In another system, an aerosol is generated pneumatically and aimed directly at the entrance of a heated capillary tube; the heated capillary exits into the vacuum system. Instead of desolvated ions entering the capillary, large charged droplets are drawn into the capillary and the droplets are desolvated while in transit. The evaporation process takes place in the capillary as well. Exiting the capillary in a supersonic jet of vapor, the analyte ions are subsequently focused, mass analyzed and detected.
This system has several disadvantages and limitations, including sample degradation, re-clustering, and loss of sensitivity. Sensitive samples are degraded due to the heat. In the supersonic jet expansion exiting the capillary, the desolvated ions and vapor may recondense resulting in solvent clusters and background signals. While these clusters may be re-dissociated by collisionally induced processes, this may interfere in identification of structural characteristics of the analyte samples. The large amount of solvent vapor, ions and droplets exiting the capillary require that the detector be arranged substantially off-axis with respect to the capillary to avoid noise due to neutral droplets striking the detector. Removing the large volume of solvent entering the vacuum system requires higher capacity pumps.
Still another system generates the electrosprayed aerosol ultrasonically, uses a counter current drying gas, and most typically operates with the electrosprayed aerosol directed at the sampling capillary. Several serious disadvantages plague this configuration. The optimal performance is effectively limited to less than five hundred (500) microliters per minute. Adequate handling of the aqueous mobile phase is problematic. Furthermore, the apparatus is complex and prone to mechanical and electronic failures.
In another commonly used system, a pneumatic nebulizer is used at substantially higher inlet pressures (as compared with other systems). This results in a highly collimated and directed electrosprayed aerosol. This aerosol is aimed off-axis to the side of the orifice and at the nozzle cap. Although this works competitively, there is still some noise which is probably due to stray droplets. The aerosol exiting the mobilizer has to be aimed carefully to minimize noise while maintaining signal intensity; repeated and tedious adjustments are often required.
While the techniques are varied with respect to the type of nebulization assist, techniques can be broadly characterized along the lines of what process is used for accomplishing ionization of the analyte. Atmospheric Pressure Ionization--Electrospray (API-ES or ES herein) and Atmospheric Pressure Chemical Ionization (APCI) differ in the ionization mechanism. Each technique is suited to complementary classes of molecular species.
The techniques are, in practice, complementary owing to different strengths and weaknesses. Briefly, API-ES is generally concentration dependent (that is to say, higher concentration equals better performance), and performs well in the analysis of moderately to highly polar molecules. It works well for large, biological molecules and pharmaceuticals, especially molecules that ionize in solution and exhibit multiple charging. API-ES also performs well for small molecules, provided the molecule is fairly polar. Low flow rates enhance performance. APCI, on the other hand, performs with less dependence on concentration and performs better on smaller non-polar to moderately polar molecules. Higher flow rates enhance performance.
At the most fundamental level, APCI involves the conversion of the mobile phase and analyte from the liquid to the gas phase and then the ionization of the mobile phase and analyte molecules. APCI is a soft ionization technique that yields charged molecular ions and adduct ions. APCI, as implemented in the hardware described herein, actually includes several distinct ionization processes, with the relative influence of each process dependent on the chemistry of the mobile phase and the analyte. What is desired is an assisted nebulization LC/MS configuration for APCI that operates in a complementary range of flow rates as does API-ES. What is further needed and wanted from the practitioner's point of view is a mass spectrometry apparatus easily and interchangeably configurable for operation in either API-ES or APCI mode with increased sensitivity in both operating modalities. What is further desired is a robust instrumentation that provides sensitive results without constant calibrating or other process interruptive maintenance procedures.